The present invention relates to synchronous machine (generator) field windings and, in particular, to the detection of electrical field ground faults in generator field windings.
A field winding in a conventional generator is generally an arrangement of conductive wires or bars in a rotor. The field windings in the rotor are generally an annular array of conductive coil bars or cables (collectively coil bars) arranged in slots around the outer periphery of the rotor. The coil bars extend longitudinally along the length of the rotor and are connected by end turns at each end of the rotor. An exciter circuit applies DC (direct current) to the coils bars of the rotor.
The insulation separating the conductor bars or end turns of a rotor of the above type may break down and cause a short circuit, also referred to as a ground condition, across one or more coils of the winding. Short circuited coils may exist when the rotor is at a standstill or, alternatively, may only occur when the conductor bars or end of the rotor are outwardly loaded by centrifugal forces. Where a ground condition persists at standstill, the condition may be detected by static tests on the rotor. However, where the ground condition arises only when the rotor is under centrifugal load, the ground condition is speed sensitive and may only be detected with the rotor turning, such as at or near its operational speed.
In one potential failure condition, centrifugal forces may cause the coil components of the rotor field to encounter the field forging of the stator. When the rotating field windings make contact with the stationary field forging a short, i.e. ground condition, may result. This type of ground condition arises only when the rotor is spinning and hence the ground condition is said to be xe2x80x9cspeed sensitivexe2x80x9d.
When a speed sensitive ground condition occurs in a field winding, locating the ground, e.g. where the rotor rubs against the field windings, has in the past been particularly difficult. The point where the rotor rubs against the generator field windings must generally be located so that a specialist can correct the short and eliminate the ground fault condition.
A conventional technique for detecting a ground fault within a generator field winding or excitation system is to use existing instrumentation in place on the generator to notify the operator via an alarm. The instrumentation, e.g. a ground fault detector, monitors pulse and continuous voltages and currents on the rotating shaft and generates an alarm when a set point is reached. The setpoint is indicative of a ground fault condition. While the alarm notifies the operator that a ground condition may exist in the field winding, the alarm does not indicate whether the ground will occur while the rotor is stationary or only while the rotor is spinning and a under centrifugal load. The alarm also does not indicate where the ground has occurred in the generator. Conventional on-line ground fault detection systems, such as disclosed in U.S. Pat. No. 3,831,160, do not indicate the location of the ground fault in a winding or the excitation system of a generator, and do not indicate whether the ground is speed sensitive.
Locating the position of a non-speed sensitive ground fault is well known. If the ground condition exists regardless of whether the rotor is moving, then the location of the ground in the windings can be determined when the rotor is stopped. However, conventional techniques for locating a non-speed sensitive ground fault in a field winding when the rotor is at a standstill are not suitable for locating a speed sensitive ground fault. If the location of a speed sensitive ground fault in a field winding cannot be located, then a specialist may have no reliable means to locate the coil bar or pole at which the ground is located. When a speed sensitive ground fault occurs, the conventional repair technique has been to perform a full field rewind. A full field rewind is a time consuming and expensive procedure.
There is a long-standing, but previously unmet need for a testing technique to locate speed sensitive ground faults in a field winding. A test to determine the location of a speed sensitive ground would reduce of the cost of repairs and reduce the repair time needed for field windings in generators.
In one embodiment, the invention is to be able to detect speed sensitive shorts to ground in the direct current (DC) components of a synchronous machine (generator).
In another embodiment, the invention is a method for locating a speed sensitive ground condition in a field winding of an electromagnetic synchronous machine having a rotor and an excitation circuit, wherein the method includes: sensing a magnetic flux generated by the field winding, while accelerating the rotor and activating the excitation circuit; detecting a cyclical aberration in the magnetic flux occurring once per revolution of the rotor; determining a position in the field winding corresponding to the cyclical aberration and identifying that position as a location of the ground condition.
In a further embodiment, the invention is a method for locating a speed sensitive ground condition in an electromagnetic synchronous machine having a rotor with coil windings and a stator with armature windings, wherein the method includes the steps of: accelerating the rotor and applying excitation to the rotor coil windings; monitoring a signal from a flux probe instrument sensing slot flux leakage; detecting an aberration in the signal from the flux probe instrument which occurs once per revolution of the rotor; correlating the aberration in the signal from the flux probe instrument to an location of the rotor, and identifying the location of the rotor correlated with the aberration as a source of the speed sensitive ground condition.